Legend
has it that the Vikings used a navigation device called the sunstone to
tell the direction of the sun, even on a cloudy day. The device was dismissed
as myth ... until today:

... experiments have shown that a crystal, called an Iceland spar,
could detect the sun with an accuracy within a degree – allowing
the legendary seafarers to navigate thousands of miles on cloudy days
and during short Nordic nights.

Dr Guy Ropars, of the University of Rennes, and colleagues said "a
precision of a few degrees could be reached" even when the sun
was below the horizon.

An Iceland spar, which is transparent and made of calcite, was found
in the wreck of an Elizabethan ship discovered thirty years ago off
the coast of Alderney in the Channel Islands after it sank in 1592 just
four years after the defeat of the Spanish Armada.